Posted
by
kdawsonon Monday March 01, 2010 @02:46AM
from the bend-me-shape-me dept.

strredwolf writes "Caltech has released a flexible solar array that converts 95% of single-wavelength incandescent light and 86% of all sunlight into electricity. Instead of being flat-panel, they stand thin silicon wires in a plastic substrate that scatters the light onto them. The total composition is 98% plastic, 2% wire — the amount of silicon used is 1/50th that of ordinary panels. So as soon as they can get these to market, solar could be very viable and cheap to produce."Update: 03/01 21:02 GMT by KD: Reader axelrosen points out evidence that the 80%+ efficiency figure is wrong. MIT's Tech Review, in covering the Caltech announcement, says that the new panel's efficiency is in the 15%-20% range — which is competitive with the current state of the art. And the Caltech panel should be far cheaper to manufacture.

Holy balls.
If this article is spot on, they've doubled the efficiency of the current technology (which converts at about 40%) AND done it in such a way that the stuff is cheaper to manufacture AND made it flexible.
This is the sort of thing that can have a real (and probably positive) impact on the world we know. Amazing.
The only remaining question (I didn't see anything about it in TFA) is how durable this stuff is compared to the current panels.

It sounds like the summary here is overstating the efficiency a bit. The numbers are for the absorption efficiency, not the overall conversion efficiency.

'The light-trapping limit of a material refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. "We've surpassed previous optical microstructures developed to trap light,"..The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons--in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater. "It's an important advance."'

It looks like the overall efficiency is still very very high while using minimal resources. This is exactly the kind of innovation the U.S. needs for carbon-friendly jobs.

There are many different factors that go into making a high efficiency solar cell. You need a front material that has very high transmittance/low reflectance at incident angles, a high absorption semiconductor, a high photon to carrier generation rate, high/easy carrier collection from the semiconductor, and broad spectrum conversion. These silicon-wire arrays appear to have high absorption and high carrier generation, but thats only part of the story. The other issue is that silicon misses out on a fair chunk of the solar spectrum. Anything after 1100nm is not converted, its simply below the bandgap.

The title of this post and the article is incredibly misleading and very annoying/frustrating to someone who's been working on solar technologies for a while. Don't get me wrong, I think this is a very cool thing, sounds like they have to potential to make very cheap cells, but approach, let alone surpass, current multijunction cells (30-40% eff.) they will not.

from what TFA says these new panels can work with dispersed light and that is why it is efficient. This means exact positioning and direct light is not longer a limiting factor. OTOH that these things can be produced & installed cheaply would mean that they do not have to sustain 20years of sun to be economical something that currently used solution requires.

1 Photon to one electron is only half the story. If the photon has more energy than the electron then there is a loss. The electron has a fixed energy (band gap) and the photons *must* have that much energy or more before it works at all. There are other details too, in silicon its not a direct band gap, so each photon cannot just eject a single electron, it must also emit a phonon (heat). Silicon has a theoretical maximum efficiency (electrical) of about 29-30% IIRC in sunlight (thats at 100% quantum efficiency for all photons at and above the band gap).

Seconded. I sold a '94 Saturn last year that had been parked in the Arizona sun for many years. (Got rid of it due to multiple electronics failures and an engine oil leak that'd not be worth it to fix). Survived the sunlight just fine.

"When was the last time you find any plastic that can last 10 years under the sun?"

My plastic garbage bins have spent at least a decade out in the Aussie sun. A lot of plastic that you find in throw away stuff these days has been deliberately engineered to be bio-degradeable due to pollution concerns in the 80's. The older non-biodegradable stuff has formed a large "islands" in the North Pacific and North Atlantic.

I'd also be very interested to know whether this can be mass produced, and for how much money. They say they are currently working on cells one square centimeter in size.

They're currently working on scaling it up, but arranging these nanowires in a large array, making the electrical connections, and filling with the polymer and scatterers sounds like it will be hard to mass-produce, even if the materials cost is not as high.

'We have shown the optical absorption efficiency and charge carrier collection efficiency of a silicon wire array cell is comparable to a conventional silicon cell, but a wire array cell uses up to 100 times less silicon due to enhanced light-trapping effects,' says Atwater. Significantly, the wire arrays absorb infrared light more efficiently that conventional silicon surfaces, further improving the performance of the new device.

So the gist is that it's more efficient because it converts infrared, uses some type of clear polymer with alumina "reflector particles" in place of 99% of the expensive (doped) silicon, and is flexible and therefore easier to manufacture.

Turns out the only benefits to this are the flexibility and low cost (which are good, sure, but not that exciting).According to their letter to nature.com this "also may offer increased photovoltaic efficiency", _may_ suggests to me there probably isn't any significant improvement.

For anyone wondering why high absorption and a high QE don't necessarily result in high energy conversion (like I was a few hours ago) it's because 30% of the photons have insufficient energy to free an electron in silicon, and most of the rest of the photons have more energy than needed to free an electron, so any excess energy beyond that required to free a single electron is wasted as heat.

No, the only remaining question is: When, if at all, will it actually come to the market. Meaning when will I be able to go to an electronics shop, grab it with my own hands, buy it, and then use it?

Until then... anyone can just make up stuff, put it somewhere trustworthy, and then let the trust-relationship-machine do the rest. (Not saying this is the case here. Just opening the mind.:)The thing is: I got absolutely zero proof that any of this is not made up. And I don‘t trust “the news”

BP is an energy company at this point, not simply an oil company. They have, more than the other huge oil companies, realized that they need to diversify, so they have moved aggressively into natural gas, solar, wind, and geothermal. Yes, they still get most of their money from oil, and will continue to focus on it as long as it is making them money. But they are hedging their bets and putting large amounts of money into research to be well positioned for whatever comes next.

As far as I can figure from the article what is says is 95/86 of the light is absorbed, it doesn't say that all of this light is converted into electricity as is stated here on Slashdot.
That is also impressive numbers and very interesting, but my guess is that the efficiency of the solar panel is going to be a lot lower than those numbers posted on the parent, most likely at least a factor 2 lower.

Yes, just like any other dark panel you leave in the sun. Except not as hot, because some of the energy is being exported as electricity. So unless they're flammable at really low temperature we'll probably be okay.

There was a bit further down that said conversion to electricity was 90-100% of absorption. That means a worst case efficiency of 77% of incident sunlight, which is still a staggering improvement over standard cells.
I for one welcome our new silicon-wire overlords.

> up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight.
It says "up to". Which means that the worst case could actually be zero and the numbers are actually meaningless. Read more carefully before welcoming your new overlords.

That 90-100% conversion efficiency isn't the whole story, either. That term is what's referred to as quantum efficiency. A shorthand way to think of Q.E. is to consider the probability that an absorbed photon will create an electron-hole pair. But that isn't the same as the electricity harnessed; it is only a pre-requisite.

The quantum efficiency of existing solar cells is also very high - approaching 100%. But a large fraction of those electron-hole pairs quickly recombine within the semiconductor.

The problem with current panels isn't the efficiency. More efficiency is welcome but the real problem with solar panels is the cost. It takes too many years to recoup the very heavy initial investment. If the price can be made such that the panels pay for themselves with 2 or 3 years then they make solar power a real alternative to the grid.

For appeal to common users, and also for appeal to producers.
Now, solar is limited by two big things:1. total cost (panels are expensive, so few people buy them, so few people produce them, so they are more expensive than it could be)2. the Return on Investment is low (extreme cases - 10 years, but typically more than 20).

If a cheap production method can be devised, this will open the market to many buyers (many people don't even consider buying a $25,000 solar panel system, but will buy in a heart beat a $2,500 solar panel system).
Also, a cheap production method will allow (hopefully) a quick panel production ramp up)

None of what you just said contributed to the conversation in anyway. All you did was repeat the GP's statement of the problem in more detail.

His point was that nuclear and hydro (and I'll add coal to that list too) power plants take longer to recoup their initial investment, so there's no reason for this to be a problem for solar power other than stupid reporters repeating the myth that it takes to long to pay for itself, and thus people actually believe it.

Because unlike nuclear, solar is a system that can be deployed on decentralised on many homes. Expect most poeple dont stay in the one home for 20+ years, so it's very hard to justify the investment.

If they can get the costs down, more poeple will buy this, just like solar how water and insulation. Not to mention rural/remote and 3rd world installations. The potential market for small systems is huge.

most poeple dont stay in the one home for 20+ years, so it's very hard to justify the investment.

Except installed solar increases the value of the house. If you live in a house a few years and have solar panels installed when you move in, when you leave it will be mostly paid for and you get more from the sell. This is even more true in California with it's high electricity costs.

If they can get the costs down, more poeple will buy this, just like solar how water and insulation.

I think it's because solar can be done at a household level, which nuclear and hydro can't. As the break-even time goes down, more homeowners are able to say, "I'll install some of those solar panels", and have the cash back relatively quickly.

However, getting the money together to buy and install the solar panels is all on me, the homeowner

No it's not. I see/hear ads from solar power rental places all the time (on local media no less, but then again it is Los Angeles). They will do the full install at no upfront, then charge you amortized payments--if the payments are less than what you save on electricity (which their ads claim will usually be the case, for what that's worth, I have no idea if that part's true) then they pay for themselves on day one.

So if financing a nuclear power plant is economically attractive, then so should financing consumer solar panels. In fact if they are lightweight an easy to install, it'd be a lot more feasible to repossess and resell them, although that means you'll probably need to insure them against theft.

That said, I'd bet the problem is with consumers rather than banks. Most houses wouldn't be able to generate enough power to go off the grid, and the payback time doesn't justify the aggravation of having another

"The break-even time for nuclear is over a decade, and it's pretty long for hydro projects too. So why do we insist that solar has to turn a profit Real Quick Now?

It's important if you want Joe Home-owner to shell out the initial investment, not so much if it's a corporation setting up a plant to generate hundereds of megawatts. That being said TFA claims this design uses 1/50th the amount of silicon crystal which is the expensive part in existing cells. Not only would it be a lot cheaper to make the new

The article adresses that issue up front, stating explicitly that 90-100% of the absorbed light is converted into electricity. From the article:

Atwater and his colleagues--including Nathan Lewis, the George L. Argyros Professor and professor of chemistry at Caltech, and graduate student Michael Kelzenberg--assessed the performance of these arrays in a paper appearing in the February 14 advance online edition of the journal Nature Materials.

The article states a quantum efficiency of %90-%100. That is the rate of photons converted to electrons. So you have a high rate of absorption and a high rate of conversion. 77% total efficiently (see below) of course they have only made them 1cm square so far.

That’s explained trough simple quantum physics and energy preservation. It must go somewhere. It can’t just “vanish”. And since absorption means it gets an electron out of the atom and moving, it is equal to electricity.

I can only recommend to learn a bit about quantum physics. Not necessarily the math, but the rules/laws of it. Useful and fun.:)

Meanwhile, Germany (where it is always cloudy, and where the government recognizes the need for renewable energy) is pushing solar like crazy, and Arizona (where it is always sunny, and where the governor has no conception of future beyond a few years) is burning coal.

I beg to differ. This is exactly what we should be using our oil reserves for: building up a supply of renewable energy. Look at it this way: we can burn our oil; or we can use it to create systems that will generate energy for us, without needing further input of oil.

I'd dearly love to see us in a world where we no longer need to burn oil or coal for energy, or if we do need to do so, we use oil we've produced ourselves - using only water and carbon dioxide as the essential inputs. On that day, we will have overcome one of the major problems facing our society today.

We'll never run out of plastic. Don't forget that "oil" came from biological sources. It'll be more expensive than just pumping the stuff out of the ground, but as long as there is life on Earth we'll be able to produce all the polymers we need.

Last time I checked you're a blithering idiot who talks out of his ass.

Does silicon grow on a beach? In a manner of speaking...

However, the factories that process raw silica into high grade silicon for semi-conductor production are in short supply, and this has driven up the price of silicon. Silica is cheap, and every where. Silicon is manufactured, and currently not cheap (enough for widespread solar panels).

You hook a lightbulb up to a solar panel, and it will keep glowing forever. Obviously this has to be done in a completely sealed box so that none of the light escapes, so you are forbidden from checking that the light is still glowing.

Some would say it's useless, but it improves the quality of life of physicists' cats quite dramatically.

That's all I really want to know. If I can put them on my roof for a reasonable price, I'll be one happy wanker, but lab situations just don't necessarily translate so well into real life.

Not having read the article I don't know whether it was mentioned.

Assuiming that production of these is not too difficult, this seems like a very good way to produce power for you and at least one neighbour just by tiling one roof (I'm pulling the figures for that calculation out of my ass, so if you want to comment on tha

This is interesting work, but it is in a very immature stage of development. They seem to be no where near demonstrating a practical solar cell, and speculated conversion efficiency numbers like 86% are laughable. One of the fundamental limitations of a cell based on Si wires is that the higher a photon's energy is over the bandgap of Si, the more energy is lost as heat. I believe the theoretical maximum conversion efficiency for a Si solar cell is around 30%, and commercially viable cells are limited to around 20% because of practical issues in creating solid state cells such as making electrical contacts to the device, the high cost of making higher efficency (20+%) Si cells. This work doesn't begin to address such issues. I think it is unfortunate that over-hype like this can take luster off of progress in photovoltaics that seems less spectacular but is much closer to practical realization.

firstly, kdawson your a tard, they aren't 86% efficent at converting light into electricity, merely at absorbing light. the 2nd warning bell for me is this - "The next steps, Atwater says, are to increase the operating voltage " - this sounds to me like they can't produce any meaningful voltages out of these, which is the exact same fail as every other flexible solar panel ever touted. infact they carely avoid talking about it's electrical output at all in TFA.

Here's the actual scientific paper, "Predicted Efficiency of Si Wire Array Solar Cells". [caltech.edu] That's by the same authors mentioned in the press release. While the thing does trap most of the light hitting it, only a fraction of the energy in that light is converted to electricity. In fact, this thing is currently less efficient than the better commercial solar cells.

So, an interesting development, but no big breakthrough. There's a claim that it might be a cheaper way to make solar cells, but everybody who comes up with a new design makes that claim. (Nanosolar comes to mind; their technology is supposed to be cheaper, but so far they've spent half a billion dollars and apparently have only produced sample panels.)

So, an interesting development, but no big breakthrough. There's a claim that it might be a cheaper way to make solar cells, but everybody who comes up with a new design makes that claim. (Nanosolar comes to mind; their technology is supposed to be cheaper, but so far they've spent half a billion dollars and apparently have only produced sample panels.)

From Nanosolar's website, it sounds like they've been shipping panels commercially for the last two years, and that they have panel assemblies in both the US and Germany...

From Nanosolar's website, it sounds like they've been shipping panels commercially for the last two years, and that they have panel assemblies in both the US and Germany...

Yes, from Nanosolar's web site, it sounds that way. But as of 2009, "not one Solarply cell has been held yet in the hands of a consumer". [buildingwithearth.com] There are no reports of actual Nanosolar installations. Supposedly they're building big solar panel installations for utility companies. So where are the announcements from those utility companies

Minor point, but that's the wrong paper. Here's [nature.com] the paper you want (may require subscription to Nature Methods). You're still correct, by the way. The researchers don't directly state conversion efficiency in their paper. They mention that above-bandgap photon absorption is roughly 85%, which is on par with current commercial PV's. They also mention that the quantum efficiency is 0.89 for the array. Unfortunately, conversion of photoelectrons to actual usable electricity is the main efficiency bottleneck in solar energy. Electron-hole pair recombination and parasitic absorption by impurities, among other things, chew away the efficiency of a solar cell in a hurry.

The take-home message from the paper, as far as I can tell, is that the researchers showed that one can achieve performance comparable to commercial solar cells by using 1% of the expensive ultrapure silicon used in current PV's.

PhD candidate doing my research in new materials for photovoltaics here.

I'm sick and tired of all this mis-reporting. These are NOT 86% efficient cells. If they were, (and they were inexpensive) it would be the greatest discovery in 50 years and it would have been all over every newspaper in the world 2 weeks ago when this paper was published.

They simply absorb 86% of light that hits them. When you say a cell is X% efficient without qualifying it, it's taken to mean power conversion efficiency [PCE] (optical power in/ electrical power out) That and dollars per watt are the numbers that really matter. Read the Nature Materials paper that drove this and you'll see that theory says this design could be up to 17% efficient. That compares unfavorably to mid to high-end commercial cells on the market today.

I'm not saying that this research is a worthless endeavor, maybe they can hit the maximum theoretically possible PCE and keep the cost down. That might have real-world impact.

The caltech news brief quotes Atwater (the PI for this research) as saying that the photons are not only absorbed, but they're also convertedto charge carriers (which is a good step). The problem he doesn't mention here is, these charge carriers loose all their energy (voltage) before they exit the cell. Solve that problem and we've got a winner.

The fundamental issue with nano-structured designs like this is the surface area of the P-N junctions in them. Large surface area means high dark current which means low voltage output. Low voltage output means low PCE. Unfortunately, nothing in this research solves that problem.

86% collection efficiency? Holy cow, that's amazing. Now, if we can just electrolyze water cheaply enough for fuel cells to solve the time-of-use problem, we could free up megatons of metals that currently make up the power grid for other uses.

photons with energy less than the bandgap of the conversion material will not be converted to electrons. photons with energy greater than the bandgap will only convert at the bandgap energy. the high effieincy multijunction cells attempt to address this. multi-exciton generation can happen if the photon is several times the bandgap energy, and there is some hope that quantom dot cells will be able to achieve high ef

It's EQE is 77-85% (above the band-gap).It's IQE is 90-100% (above the band-gap).But it's energy conversion is similar to other commercial panels; about 20%.

High absorption and high QE is not enough to get high conversion rates.You still have the band-gap (the minimum frequency which a photon needs to be able to free an electron from silicon) which excludes up to 30% of all photons, and almost all photons above the band-gap which do free an electron have more energy than is necessary to do so, so the excess

"as soon as they can get these to market, solar could be very viable and cheap to produce." And if a frog had wings his ass wouldn't bump the ground when he hops.

I appreciate Slashdot acting like an old Popular Mechanics here, but I wouldn't get too excited just yet. As somebody pointed out in another forum, when you compare ethanol with gasoline in terms of efficiency, if all we had was ethanol primarily from "corn" (U.S. term, UK term is "maize") and then someone invented gasoline, we would be raving about the improvement in efficiency and economy. IOW, I will believe cheap, efficient solar power when I see it on the neighbor's roof. Until then, this is one more expensive quest for a pot of gold at the end of a rainbow. In the meantime, we could be practicing more energy efficiency.

FWIW, I knew W was full of crap with that whole "hydrogen economy" nonsense back around 2005. That was an absurd sop to deflect a little criticism that he was as much a tool of Big Oil as his Old Man. Make note that I served in Iraq during Operation Desert Storm and when it was over, George H.W. Bush was sitting on a 91 percent approval rating based on a war we had to fight to maintain a steady supply of petroleum for the Western Powers and Japan. From the desert, I wrote my Senators and lobbied them to get a bill going to get us to start weaning off Mideast Oil. That S.O.B. Bush didn't raise a finger, nor did our Congress and eventually Western wealth transfer begat Osama Bin Laden, 9/11, Iraq War II, and Afghanistan. Wouldn't you think a 91 percent approval rating might have been enough political capital to change things a little? It may even have made Bush the Elder seem like the President of the U.S.A. instead of President of the New World Order since he rightfully earned a reputation for being allergic to domestic policy. His detachment had a lot to do with getting booted in '92. A review of the stock market back in '90 - '91 reveals that Big Oil shot up and helped a lot of folks in that business recover from the very hard times they went through in the late '80's. Though I was a conservative and a combat veteran, I campaigned for Bill Clinton in '92 as I was so disgusted with Bush the Elder. Still am. God save us from another Bush.

Most of us know in our heart of hearts that our troops are in Iraq and Afghanistan because of the continued grip the Mideast has on Western economies. In World War II, the U.S. national speed limit was 35 mph and gasoline was rationed with coupons. This was done to make sure the military had plenty of fuel. If some shared sacrifice was called for now, I think most Americans would grumble, but go along with it for the sake of untangling from the Iraq and Afghan Wars. How about bringing back the 55 mph speed limit of the '70's and '80's? What about a tax based on the weight of a vehicle? If we cut back on petroleum use, we help our independence and the environment at the same time. Now that's what I call "conserve-atism".

If you want to see what needs to be done about our dependence on petroleum, just look for the occasional Charles Krauthammer piece on it. He makes the same recommendations about every 5 years, the centerpiece of which is a flexible tax on gasoline that seeks to wean us off cheap oil while keeping the price of gasoline fairly steady.

The original article is poorly written (no, not even close to 86% you stupid twats) and kdawson is equally foolish for echoing this garbage. This is why this site sucks. Brain-dead slashdot editors, time and time again, post shitty articles that make extraordinary claims which end up being completely false or misleading.

Slashdot, please think of the children that will be disappointed by this article.

Us grownups can see through the PR-speak, but kids can't.

We can see that this loose talk of high efficiencies is just that-- only part of the story.

It's swell that these gizmos have a 97% absorption efficiency, but that's only the front end.

The actual cell, which converts the light to electricity, is no different-- about 16% efficient, dueto the many mismatches in energy levels and the unavoidable phonon products.

Plus the business about needing less silicon is not spreadsheet-worthy. The actual bulk silicon is not a large part of the cost.Even if they got the silicon usage down to 0%, the cost would not come down very much if at all.

Also the economic predictions are unrealistic. Nothing that's better has ever sold for less than 5% under the price of the competition. No company can afford to leave money on the table.

Right now, solar cells are so expensive, they take something like 15 or 20 years to pay for themselves, so most property owners don't see a big incentive. Lower that price to 10 years or 8 years, or even lower, and suddenly the demand for these things will skyrocket.

The payback period for solar is already under 10 years. New Jersey [calfinder.com] has a payback period of 1.5 years, "New York and Delaware are next in line with payback in 3-6 years, and California, Maryland, Massachusetts and Wisconsin all tied for fourth a

Ever heard of polycarbonate? They use it to produce composite bulletproof window panes, safety shields for industrial machinery, impact-resistant safety glasses, underwater portholes, etc. It does degrade somewhat under UV light, but then, you can just put an UV filter on top of it, it's not going to be a problem for the panel itself. And there are other transparent plastics with very good properities for this application.

Yes and no. Yes, UV is the most energetic light, but there isn't much of it in sunlight at the earth's surface. Most of it has been lost to ozone, Rayleigh scattering, etc. There's enough to give you a sunburn, but no, in terms of the actual amount of power in sunlight at the earth's surface, it's less than 5%. Filtering it to zero to obtain much longer PV panel lifetime is generally a net economic benefit in terms of TCO.

Collection efficiency (which is what TFA is claiming to be 86%) vs. conversion efficiency (that 40% number you remember) is what you're missing, but from other articles on the technology it appears that the conversion efficiency for these cells should be higher than existing designs:

The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons--in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater. "It's an important advance."'

Which could give them ~78% conversion efficiency, still nearly double over the best cells currently.